NASA news indicated that an intense magnetic storm on the sun had caused it. The sun's hydrogen atmosphere is continuously boiling off the sun at high speeds, creating solar winds whose charged particles strike the earth. Sunspots, areas of increased magnetism on the sun's surface; coronal mass ejections, violent bubbled eruptions in the suns outer corona; and solar flares, abrupt outbursts of energy and matter; are the main releasers of large amounts of energy contained in ionized gas particles that produce enhanced auroras on earth.
These loads of energy, which can be as massive as a mountain or as powerful as a billion tons of explosives, travel two to four days before hitting the earth's own magnetic field. Solar winds compress the magnetic field on Earth's side facing the sun, the magnetosphere, creating a build up of pressure.
On the other side of Earth, charged particles enter the tail end of the magnetosphere, past Earth, and the increased pressure from the magnetosphere produces an energy voltage between this tail and the Earth's magnetic poles (the set up is similar to a batterys two charges connected by a wire).
Because of the voltage, electrons move to the poles at high speeds, pushing numerous electrons into the earth's ionosphere. Electrons then collide with gases which become energized and release this influx of energy in forms of light and more electrons. This phenomenon forms rings around the magnetic poles; it shifts due to changes in solar winds and the electric currents causing the aurora.
Therefore, what you see in the sky is actually a small portion of this ring. The type of gas with which the electrons are colliding determines the color of light emitted. For example, sodium emits dark yellow, neon emits orange, oxygen lower in the atmosphere makes a yellow-green (the most common color), nitrogen emits red and hydrogen and helium make blues and purples.
Some parts of this color spectrum are easier for the human eye to see and although auroras do happen in the daytime, they can typically only be seen at night.